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Port Address Redirection
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The last example showed PAT being carried out dynamically by the address translation device In some situations, however, this will not work For instance, your ISP might assign you a single public IP address You need to use this with PAT to allow inside users to access outside resources However, you have a problem if you want outside users to access an internal service, such as a web server Dynamic PAT, unfortunately, won t work in this situation Another solution to this problem is available: static PAT, often called port address redirection (PAR) Let s look at a simple example to illustrate how PAR works Assume that your ISP has assigned you a single public IP address: 1991991991 You need to use this address for inside users to access the outside world, but you still need the outside world to access an internal web server With static PAT, you set up your address translation device to look not only at the destination IP address (1991991991), but also the destination port number (80 for a web server) You create a static PAT entry such that when the address translation device sees this combination of address and port numbers, the device translates it to the inside local IP address and, possibly, the port number used for the service on this inside device
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For example, assume you are given the network shown in Figure 23-1 In this example, your ISP has assigned you a single IP address, 2002002001, and this address must be configured on your router s S0 interface This presents a problem in this example, since you have an internal web server that you want external users to access Port address redirection can be used to overcome this problem You would set up a static PAT entry on your router that would take TCP traffic sent to 2002002001 on port 80 and redirect it to 10111 on port 80 Without PAR, the router would try to process the web connection itself since this IP address is assigned to its own local interface
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Port address redirection allows you to redirect application traf c
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As mentioned at the beginning of this part of the chapter, address translation devices are typically used to give you an almost inexhaustible number of addresses as well as to hide your internal network addressing scheme Another advantage of address translation is that if you change ISPs or merge with another company, you can keep your current addressing scheme and make any necessary changes on your address translation device or devices, making your address management easier Another big advantage that address translation provides is that it gives you tighter control over traffic entering and leaving your network For example, if you are using private addresses internally, all traffic entering and leaving must pass through an address translation device Because of this restriction, it is much easier to implement your security and business policies
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Advantages of address translation include conservation of public addresses, protection of resources with
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private addresses from external devices, and no need for readdressing of devices when switching from one ISP to another
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23: Address Translation
Disadvantages of Address Translation
Even though address translation solves many problems and has many advantages, it also has its share of disadvantages Here are the three main issues with address translation:
Each connection has an added delay Troubleshooting is more difficult Not all applications work with address translation
Since address translation changes the contents of packets and, possibly, segment headers, as well as computing any necessary new checksum values, extra processing is Remember the required on each packet This extra processing, disadvantages and limitations of address obviously, will affect the throughput and speed translation of your connections The more packets that pass through your address translation device needing translation, the more likely your users will notice the delay Therefore, choosing the appropriate product for address translation becomes very important Whenever problems arise with connections involving address translation, it is more difficult to troubleshoot them When troubleshooting, it becomes more difficult to track down the real source and destination of a connection you have to log into your address translation device and look at your translation tables And if the packet is going through multiple layers of translation, possibly at both the source and destination sites, this can be a hair-pulling experience Also, even though one of the advantages of address translation is that it hides your internal addressing scheme, it also creates security issues an external hacker can more easily hide his identity by sending his packets through a translation device or multiple translation devices, trying to hide his true IP address Probably the most difficult issue with address translation is that not all applications will work with it For instance, some applications embed IP addressing or port information in the actual data payload (such as FTP, for example), expecting the destination device to use this addressing information in the payload instead of what is in the packet and segment headers This can pose a problem with address translation, since address translation, by default, doesn t translate data payload information, only header information Multimedia and NetBIOS applications are notorious for embedding addressing information in data payloads In some instances, some vendors address translation devices can detect this process for certain applications and fix it
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